1. Field of the Invention
The present invention relates to a process for manufacturing a ceramic wiring substrate or board and in particular to a process for manufacturing a hybrid multilayer ceramic wiring substrate having a low dielectric constant on which ultra high speed LSI chips are mounted.
2. Prior Art
Semiconductor devices such as ICs and LSIs have heretofore been mounted or packaged on printed circuit boards of glass epoxy or alumina ceramic substrates. In association with an advance in high integration, reduction in size and speeding-up of semiconductor devices, demand for mounting substrates which meet the requirements for high density wiring, high speed transmission and high frequency and high heat dissipation has been increased.
Conventional printed boards have problems in through-hole plating properties, machining properties, multilayer bonding ability and thermal deformation at high temperatures, so that they have a limited density. Accordingly, the printed board has not been yet put into practical use as a high density mounting substrate. In contrast to this, the ceramic substrate has a higher potential for high density mounting substrate.
Since the alumina substrate should be sintered at elevated temperatures not less than 1500.degree. C., a conductor material for wiring which is simultaneously sintered or fired is limited to refractory metals or high melting point metals such as W, Mo and the like having a relatively high resistivity. Accordingly, if the transmission loss of pulse signal is considered, reduction in size of wiring pattern would be restricted.
On the other hand, since the propagation delay time of pulse signal is proportional to the square root of the dielectric constant of the substrate material, the low dielectric constant of the substrate material is essential to achieve a high speed transmission. However, the alumina substrate has a relatively high dielectric constant of about 10.
It is a low-temperature sinterable multilayer ceramic substrate that has been developed. The insulating material includes a material such as composite materials of ceramics with glass and crystallized glass. Since the above materials can be sintered at a temperature not higher than 1000.degree. C., low melting point metals, such as Au, Ag-Pd, Cu or the like, having a low resistivity can be used as a wiring conductor material It is possible to lower the dielectric constant of the insulating material not more than 5 by choice of ceramics or glass having a low dielectric constant Since a green sheet lamination technique can be used, it is possible to make three-dimensional wiring, which is very advantageous to obtain a high density.
Formation of the wiring pattern in the green sheet lamination technique is generally performed by a thick film screen printing method. The lower limit of the width of line and space in mass production is 75 .mu.m. Accordingly, a new approach and structure is necessary for micro wiring.